Internal combustion engines include one or more pistons connected to a crankshaft by a connecting rod. During the combustion cycles of the engine, the pistons reciprocate along a longitudinal axis within a combustion chamber defined by the walls of a cylinder to drive the rotation of the crankshaft. For example, in a four stroke combustion cycle of a premixed spark-ignited or dual fuel engine, the piston may move from top dead center to bottom dead center to draw in a mixture of fuel and air into the combustion chamber through the intake valve. Following air/fuel intake, the intake valve closes and the piston moves from bottom dead center to top dead center to compress the air/fuel mixture. Subsequent initiation of combustion of the air/fuel mixture with a flame causes the gases to expand which forces the piston to move from top dead center to bottom dead center. A spark is used to initiate combustion in premixed spark-ignited engines, whereas a small amount of diesel fuel (“pilot” fuel) is ignited by compression to initiate combustion in premixed dual fuel engines. Following combustion, an exhaust valve is then opened, and exhaust gases are evacuated from the combustion chamber as the piston moves from bottom dead center to top dead center, allowing the combustion cycle to repeat.
A piston includes a piston head having an upper surface facing the combustion chamber. Around the periphery of the piston head are a plurality of annular ring grooves that receive piston rings which form a seal between the combustion chamber and the crankcase. In addition, a plurality of “lands” extend between and above the annular ring grooves to confine the piston rings in their respective grooves. Above the top piston ring is a “top land” which extends to the upper surface of the piston head. The piston also includes a skirt that includes a pin bore that receives a wrist pin for connecting the piston to the connecting rod. The wrist pin may serve as a bearing on which the piston rocks from side to side as it reciprocates in the combustion chamber.
In prior art designs, the top land of the piston may have a cylindrical shape with a smaller diameter than the engine cylinder, such that the top land is separated from the cylinder walls by a gap which is accessible to air and fuel in the combustion chamber, but is inaccessible to the propagating flame due to heat loss to the closely-spaced metal surfaces. Moreover, as the piston rocks about the pin bore axis (i.e., in a direction concentric to the pin bore axis), the top land may more closely approach or contact the cylinder wall, temporarily creating blocked crevices containing trapped hydrocarbon fuel that cannot be combusted by the propagating flame. As a result of flame inaccessibility to hydrocarbon fuel trapped in such top land crevices, emissions of unburned or partially unburned hydrocarbons may be undesirably increased, contributing to the greenhouse gas footprint of the machine. Hydrocarbon emissions from top land crevices is a problem associated with premixed spark-ignited and premixed dual fuel engines which, unlike diesel engines that ignite fuel through compression, rely on a propagating flame for combustion.
One approach to reduce hydrocarbon emissions from top land crevices, as described in Canadian Patent Application Number CA 2863036 A1, involves the introduction of an annular chamfered edge around the periphery of the top land to provide a conically-shaped top land. The chamfered area of the top land provides clearance for piston rocking and reduces the crevice volume in the direction of rocking, resulting in reduced unburned/partially burned hydrocarbon emissions. However, while effective, the conical/annular chamfered top land geometry unnecessarily augments the crevice volume near regions of the top land where additional clearance for piston rocking is not needed (i.e., directly above the wrist pin bore). By increasing the crevice volume that is difficult for the combustion flame to access, more unburned or partially burned hydrocarbons may be emitted, counteracting at least some of the emission reductions obtained with the annular chamfered edge.
Thus, there is a need for improved piston top land geometries that help reduce unburned or partially burned hydrocarbon emissions from crevices.